Activated carbon with magnetic properties has the advantage of being easily separable from a system. The idea to precipitate or crystallize a magnetic component into the porous network of an already activated carbon is straightforward and has been successfully applied. Incorporating iron components together with a precursor during the synthesis is the preferable procedure to obtain a functional and magnetic activated carbon. However, such incorporation reduces the surface area and pore volume of the activated carbon, and the magnetic component is only loosely integrated and may leach.
Hydrothermal carbonization can provide an effective process for introducing nanosized iron components into carbonized material. Hydrothermally carbonized (HTC) biomass is a stable organic material with many prospective applications. Its associated carbonization process has high carbon efficiency, it is exothermic, and it has minor associated release of greenhouse gases. Baccile et al. determined the molecular nature of HTC glucose by carbonizing glucose that was enriched in 13C and studied the product with multidimensional solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.9 From such data they concluded that HTC biomass was on an average consisting of a heavily cross-linked polymer with mainly furanic moieties. Recently they showed that when nitrogen was present in the organic raw material, it was included in the cross-linked HTC biomass.10 
Activation of regular biomass into activated carbon can be performed physically or chemically. Physical activation is typically performed by treating various carbonaceous matter with air, carbon dioxide or steam at high temperature. Chemical activation is performed at elevated temperatures with addition of compounds such as KOH, ZnCl and H3PO4. Sevilla et al. chemically activated HTC biomass produced from eucalyptus sawdust with KOH at a temperature of 800° C.12 Liu et al. activated HTC biomass prepared from pinewood sawdust and rice husk with carbon dioxide and reached specific surface areas of 569 m2/g.13 
Waste biomasses often have environmental problems. Composting of horse manure releases climate gases and does not fully destroy pharmaceutical residues and pathogens that may leach into the environment. Grass cuttings releases both nitrous oxide and methane to the atmosphere, which both are greenhouse gases. Waste from beer production has a low economic value and is mainly used as animal food. Biosludge can seldom directly be used as fertilizer and those from pulp and paper mills have insignificant commercial values.
U.S. Pat. Nos. 7,429,330 and 7,879,136 disclose a process for preparing magnetic activated carbon by adding solid-based iron oxide to a coal/pitch mixture, and compacting it before carbonization, followed by activation into activated carbon according to existing activation processes.
US2010/0155335 discloses a process for preparing magnetic activated carbon by mixing already activated carbon with an iron solution, which has an +2 or +3 form, and followed by increasing the pH to let iron oxide precipitated in the pores of activated carbon.
US2004/0147397 discloses a process, whereby magnetic activated carbons are prepared in a traditional two step method of carbonization and activation. Soluble iron is introduced into the carbon prior to carbonization by soaking the carbon precursors into a solution of the magnetic material precursor.
U.S. Pat. No. 8,097,185 discloses a process for preparing magnetic activated carbon, whereby a carbon precursor of soft wood is soaked in a solution of a ferric salt, dried, pyrolized and activated.
There is still a need for a more efficient and cost effective process for the preparation of active carbons. There is also a need for improved magnetic activated carbon products. There are economic and environmental advantages to be able to use ubiquous and low cost biowaste biomass.